The carbonylation of methanol produces acetic acid:
Prior to 1970, acetic acid was made using cobalt catalyst. A rhodium carbonyl iodide catalyst was developed in 1970 by Monsanto. The rhodium catalyst is considerably more active than the cobalt catalyst, which allows lower reaction pressure and temperature. Most importantly, the rhodium catalyst gives high selectivity to acetic acid.
One problem associated with the original Monsanto process is that a large amount of water (about 14%) is needed to produce hydrogen in the reactor via the water-gas shift reaction (CO+H2OCO2+H2). Water and hydrogen are needed to react with precipitated Rh(III) and inactive [RhI4(CO)2] to regenerate the active Rh(I) catalyst. This large amount of water increases the amount of hydrogen iodide, which is highly corrosive and leads to engineering problems. Further, removing a large amount of water from the acetic acid product is costly.
In the late '70s Celanese modified the Monsanto process by adding lithium iodide salt to the carbonylation. Lithium iodide salt increases the catalyst stability by minimizing the side reactions that produce inactive Rh(III) species and therefore the amount of water needed is reduced. However, the high concentration of lithium iodide salt promotes stress crack corrosion of the reactor vessels. Furthermore, the use of iodide salts increases the iodide impurities in the acetic acid product.
In the late '90s, Millennium Petrochemical Company developed a new rhodium carbonylation catalyst system that does not use iodide salt. The catalyst system uses a pentavalent Group VA oxide such as triphenylphosphine oxide as a catalyst stabilizer. The Millennium catalyst system not only reduces the amount of water needed but also increases the carbonylation rate and acetic acid yield. See U.S. Pat. No. 5,817,869.
One challenge still facing the industry is that lowering water concentration in the methanol carbonylation results in increased aldehyde formation. Methods for reducing aldehyde concentration in acetic acid are known. For instance, U.S. Pat. No. 6,667,418 discloses a method for reducing aldehydes by oxidizing them with air, hydrogen peroxide and other free radical initiators in an integrated acetic acid production process at an elevated temperature. Introducing free radical initiators into an acetic acid production process is inconvenient.
U.S. Pat. No. 7,208,625 teaches a method for removing aldehyde and other permanganate-reducing impurities from an acetic acid product. The method comprises contacting an acetic acid product containing permanganate-reducing impurities with peracetic acid and an oxygen-containing gas. The method is particularly suitable for post treatment of acetic acid that contains permanganate-reducing impurities. Because the peroxide is not used in the integrated acetic acid production process, its handling is relatively safe and convenient.
Co-pending application Ser. Nos. 11/496,900 and 11/508,109 disclose methods of removing aldehyde impurities from acetic acid by reacting aldehyde with hydroxyl compounds to form corresponding acetals. The acetals are subsequently removed from acetic acid by distillation.
New methods for reducing aldehydes in acetic acid are needed. Ideally, the methods are performed conveniently and safely.